Undetermined Gull (Larus sp.)
This form will be mentioned here only for the sake of completeness, because most likely it doesn’t constitute a special species. [1]
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Only a single bone fragment is known, the proximal end of a right humerus, that originates from a large gull species, but which cannot be assigned to any certain species.
Most likely this fragment is a remain of a bird that only stayed on the island as a guest.
The most likely candidate, in my opinion, might be the Kelp Gull (Larus dominicanus Lichtenstein), which still regularly visits the islands of the Tristan da Cunha archipelago without breeding there. [1]
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References:
[1] S. L. Olson: Paleornithology of St. Helena Island, south Atlantic Ocean. Smithsonian Contributions to Paleobiology 23. 1975
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edited: 29.04.2021